Aerial feed and aerial tuning circuit arrangement



Dec. 2, 1941. N, T ET AL 2,264,718

AERIAL FEED AND AERIAL TUNING CIRCUIT ARRANGEMENT Filed April 2, 1938 2Sheets-Sheet.l

C1 C1 3 L L L L L F 421' 42e 42a 42c 42b 420 2 100w c c Fly '3 8 I L LL3 1? WWWI a 3 3 3 C3 T T T T T370000) 43- 44L i L5 Q,- L5 C L; C6- L5INVENTORS NOEL MEYER RUS T JOHN Z265 T RAMS/4) BY I g ATTORNEY PatentedDec. 2, 1941 AERIAL FEED AND AERIAL TUNING CIRCUIT ARRANGEMENT NoelMeyer Rust and John Forrest Ramsay,

Chelmsford, England, assignors to Radio Corporation of America, acorporation of Delaware Application April 2, 1938, Serial No. 199,591 InGreat Britain April 1-5, 1937 4 Claims. (c1. 178-44) This inventionrelates to aerial feed and aerial tuning circuit arrangements and hasfor its main object to provide improved arrangements wherein one or moreaerials is or are remotely situated with respect to tuning apparatustherefor, or with respect to receiving apparatus associated therewith,said a-erial or aerials being connected to said'apparatus through alength or lengths of high frequency cable.

As is well known serious difiiculties as-regards impedance matching,efiicient energy transfer and tuning are experienced when an aerial oraerials is or are remotely situated with respect to tuning or receivingapparatus associated therewith and the present invention seeks to solvethese difiiculties in a simple and satisfactory manner.

According to the main feature of the invention an arrangement wherein aremotely situated aerial is associated with a receiver or with tuningapparatus through a high frequency cable is characterised in thatsaidaerial is connected to the aerial end of said cable through arectance transformer constituted by a tapered artificial line, said lineconsisting of a plurality of sections of progressively diif'erentelectrical dimensions whereby said line matches, at one end, the aerialimpedance and at the other end the high frequency cable impedance, theimpedance transformation over the line taking place in a plurality ofsteps determined by the number of sections in the line.

The line may be arranged to give high pass, low pass, or band passcharacteristics as may be required. Further the line may be arranged tocouple a symmetrical aerial, such for example as a centre feed dipole,to an asymmetrical cable, such as an ordinary concentric tubular airspaced cable with its outer conductor earthed, or. vice versa.

Where it is required to couple two alternatively utilisable receivingaerials, one a long wave aerial and the other a short wave aerial, to acommon receiver which can' be tuned in either wave range, the inventionenables said aerials to be permanently coupled to said receiver througha common high frequency cable by providing two tapered artificial lines,one between the long wave aerial and the cable and the other between theshort wave aerial and the cable. With such an arrangement reception canbe effected from either aerial without adverse interference due to theother.

An important feature of the invention resides in improved arrangementsfor remotely tuning an aerial by tuning apparatus. which may be remoteboth from the aerial and from associated receiving apparatus andaccording to this feature a series or current tuning network isinterposed between two lengths of high frequency cable one of which iscoupled to the aerial through one tapered artificial line and the otherof which is coupled to the receiver through another tapered artificialline.

The, invention is illustrated and "further explained in connection withthe accompanying drawings wherein Fig. 1 illustrates the inventionincorporated with a dipole of the center feed type; Fig. '2 illustratesthe invention applied in an arrangement using a rhomboid aerial; Fig. 3is a circuit diagram wherein the invention is applied to a systemincorporating an ordinary medium and long wave aerial; Fig. 4 a circuitdiagram illustrating a circuit arrangement according to the inventionwhich has band pass characteristics; and Fig. 5-11 are diagrams whichare used to explain the various features and functions of the invention.

Referring to Figure 1 a receiving dipole I of Ithecentre feed type isconnected to remotely situated receiving apparatus or tuning apparatus(not shown) through a tubular concentric air spacedcable 2, '3, havingits outer conductor 2 earthed. The aerial end of this cable i .coupledto the aerial through a tapered artificial line 4|. This line consistsof a plurality of sections-in Figure 1 four sections 4a, 4b, 4c, 4d areshown though the more the sections the smoother will be the impedancetransformation-and, where a low pass effect is required each section(except that (461) nearest the cable) may consist of two sively increasein the direction of the cable. The other half of the dipole is connectedto the outer 2 of the cable through the other line wire which consistsof three series inductances L2 whose values progressively decrease inthe direction of the cable. Four shunt condensers C whose valuesprogressively decrease in the direction of the cable, are providedbetween the wires of the line as shown. It will be seen that this line4! is a low pass line presenting low impedance (a practical figure is 76ohms as indicated) at the aerial end and a higher impedance (a practicalfigure is 100 ohms as indicated) at the cable end while furthermore theline not only gradually transforms the impedance but gives a gradualtransformation from symmetrical connection (at the aerial end) toasymmetrical (at the cable end). 7

In another example in accordance with the invention and shown in Figure2 a rhomboid aerial of, say, 800 ohms, is coupled to a concentrictubular cable 2, 3, of, say, 100 ohms, through a tapered artificial line42of high pass characteristics and shown as having six sections 42a 42/.As before one terminal of the aerial 5 is connected to the cable inner 3through one wire of the line and the other terminal of the aerial isconnected to the cable outer 2 through the other wire of the line. Saidone wire of the line consists of a plurality of series condensers C1 (asshown six) progressively decreasing in value in the direction of thecable, and said other wire consists of another plurality of seriescondensers C2 (one less than in said one wire) which progressivelyincrease in value in the direction of the cable. Shunt inductances L areprovided across the line, these inductances progressively decreasing invalue in the direction of the cable. The number of inductances is thesame as the number of condensers C1 the first inductance being directlyacross the aerial and the other inductance being so connected that thetapered line consists of a plurality of sections each (except thatnearest the cable) comprising two condensers (one in each wire) and ashunt inductance. The section nearest the cable differs from the othersin having no condenser 02. Here again the impedance junction points ofthe series inductances L3 and earth. To take practical figures the line44 may present an impedance of about 3,000 ohms at the aerial end and animpedance of about 100 ohms at the cable end, for long and medium waves,presenting very high impedance at the cable end for the short wave range(-50 metres) and attenuating short waves very strongly. The second line43 consists of a series of condensers C4 of increasing magnitude in thedirection of the cable, the series being connected at one end to theshort Wave aerial 'l and at the other to the cable inner 3. Shuntinductances L4 of diminishing magnitude in the direction of the cable,are provided between the live end of the inverted V aerial l and earth,and between the junction points of the various condensers C4 and earth.Again to take practical figures, the second artificial line 43 maypresent an impedance transformer is smooth and there is transformaw tionfrom symmetrical at the aerial end to asymmetrical at the cable end.

In the embodiment of the invention shown in Figure 3 an ordinary mediumand long wave aerial 6, operable over a range of say ZOO-2,000 metres ispermanently connected to one end of the inner 3 of a concentric tubularcable with an earthed outer 2 through a first tapered artificial line44, and a short wave inverted V aerial 'l operable over a range of say15-50 meany suitable correction network or networks as known per se andindicated diagrammatically at 8) to the aerial 6. Shunt condensers Caof'increasing magnitude in the direction of cable, are provided onebetween the aerial end of the inductive wire of the line and. earth (theother wire) and the remainder between the various of about 400 ohms atthe aerial end an impedance of about ohms at the cable end for shortwaves, presenting very high impedance at the cable end for medium andlong waves and attenuating these waves very strongly.

The invention is not limited to the precise forms of tapered artificiallines already described; for example, where a band pass effect isrequired, such a line might be as shown at 45 in Figure 4 consisting ofa plurality of sections of graded dimensions, each section consisting ofan inductance L5 and a capacity C5 in series in one wire and aninductance L6 and a capacity C6 in parallel between said one wire andthe other wire, which is a simple conductor or an earth connection.

In series resonant type tuners large value inductances and small valuecondensers can be used provided that the ranges of variation of thecondensers are extended enough to give the required tuning ranges. Smallvariable condensers of excellent maximum-to-minimum capacity ratio, andvery high efiiciency have already been developed for short wave workingand such condensers are most suitable for use in carrying out thisinvention as tuning condensers. It has been found that adequate tuningrangesoften better than those normally obtainable with parallel tunedcircuitsare readily obtainable and it will be realised that the externalwiring is at a minimum and therefore stray capacities can be reduced tovery small values. The practical limit to increasing inductance andreducing capacity in a series tuner is probably set by coilself-capacities (which increase with increase of inductance) whichaffect the high frequency ends of the tuning ranges. Coil losses are attheir most serious in efiect where low frequencies are in question, e.g. the long wave broadcast band.

In addition to the advantages already mentioned tuner arrangements asabove described are readily ganged with remotely controlled tuning meansin the receiver, e. g. in the case of a superheterodyne, with a remotelycontrolled local oscillator.

In experimental practice for broadcast reception it has been foundadvantageousto interpose long. wave stopper condensers, of theorder, of1,000 micro-micro-farads in the series tunercircuits, These condensersalso facilitatecircuit matching for ganging purposes.

The tuning arrangements above described are of wide application, e. g.to commercial receivers, relay station receivers, television. receivers,hotel receiver installations. and so forth.

,There willnow be given a brief mathematical description to facilitatethe'quantitative design of a reactancetransformer constituted by anarti'ficiaT line for use in carrying out the inverttion. v

Consider a 1r section as shown in Figure 5': having: terminals PQ on theend it and: terminals- RS on the end b. Let 2- be the impedanceoi? theseries element and. YL. the admittances of the shunt elements: asindicated: in the figure, Yl and Y2 being, in general; unequal' LetZoa=characteristic impedance looking in at the end a; Zob=characteristicimpedance looking in at. the

endb. Zag=impedance between P and Q, with R and S connected together.Zbg=impedance between R and. S with P and Q connected together.Zaf=impedance between. P and Q with R and S open circuited;Zbf=impedance between R and S with P and Q open circuited. 0=the line.angle of'the' 1r section (the image transfer constant). +ifl a=theattenuation constant of? the 1r section. #:the phase change constant ofthe 1r section. Then tanh 0= For the 1r section ofFigure- 5 we have thefollowing relations:

and

(5702 I-coshz 65-1 Y1 /(ZoaZob) sinh a T Consider the case of aseeond.1: section. of. sim; 15

/B.cosh 0 l ilar configuration having corresponding con stants:" Z Ylf52 2?; zfte z ometoz If the second section is connected in tandem withthe first, there'will be no matching loss (at a specific frequency) if Z'oai=Zob.

Of the possible 'values the line. angle of the second section may take,that value where it is equal to the'lina angle of. the first section istaken (in contrast with theihi-therto usual meth- 0d of making the lineang-l'es' unequal).

Thus tanh 0 =tanh 0. 1

As the second section containszthree independent variables Z YI Yz athird relation is necessary in order that they may be independentlyspecified. Such a relation may be obtained by giving the sections equaltaper ratios, B, i. e. B :8.

It may then be established that are 1 2 B the taper ratio of thesections being also the taper factor applying to; the elements of thesecond section-i It is then possible to build up a chain of sections inwhich the. taper ratio per section is constant, the b end of eachsection matching the it end of the next. The construction of such achain is illustrated by Figure 61 In order to secure a step uptransforming action in the prototype section of Figure 5, it isnecessary that B should. be. real, positive and greater than unity; Twocases arise, derivable from the formula. iorB, viz':

These conditions are equivalent to conditions of zero attenuation; incase. (I), however, the conditions, while necessary, are not sufficient.For zero-attenuation tanh 0' must be purely imaginary, i. e. sinh 0 mustbe. imaginary and cosh HreaL. w

Ifsin'h 0 is to be imaginaryZYf+ZY2+Z YfY2 must be negative.

In (1 ZYII and ZYZ' are negative hence For cosh 0 to be realandnumerically less' than unity this condition is. also: necessary andsumcient.

2-) Z Yl and ZYZ' are negativeand less than unity; conditions necessaryand su-fiicient that tanh 6 should be imaginary.

The above: conditions define frequencies analogous to the cut-oiifrequencies of an ordinary symmetrical filter.

If the section be simplified by making the shunt arm Yl vanish, only thesecond criteria app1y ,,ie.v Yl YZ and -1. Yi Z Q -l. .Y2Z 0.

The sections then. take the; form of. Figure 7:,

Y2 being replaced by Y. The fundamental formulae then have the followingmodified form:

Z Zag Z Zbg=m an H g r-e 4 1 B B =sech Z=Zoa /(1-B) =Z0a tanh 0 /(1 B)sinh 0 cosh 0 B Zoa Zoa For a step-up action B 1, hence tanh 0 isimaginary. Since and B is positive and greater than unity, ZY will benegative and fractional.

Given the input characteristic impedance, the frequency and the taperratio, the first section is fully determined from the formulae Therelations obtained above for the 1r and T sections have been ostensiblyfor low-pass configurations. Similar formulae may be derived for highpass sections if in the 1r section Y, Zl, Z2 are written for Z, Yl, Y2and in the T section if Y1, Y2, Z are written for Zl, Z2, Y.

The principle of asymmetry (associated with atransforming action) may beapplied to chains of sections of other prototype configurations, e. g.bridge I and lattice, and to derived sections. Band pass filters havinga step-up ratio in the pass bands may also be obtained.

The number of sections adopted in any given problem will usually be aneconomic compromise dependent upon the frequency range the system isexpected to cover and the overall step-up required. The larger thenumber of sections, the smaller the step-up ratio per section and themore even the matching over a given frequency range.

The marked advantage of proceeding on the basis of the two-elementarrangement of Figure 8 (the Z arms being inductances and the Y armscondensers for a low pass line, and vice versa for a high pass line) isthat the factor A is independent of frequency, the only mismatchingrequired to be taken into account being that due to the dependence of Bon frequency.

The frequency usually taken as the matching frequency is the logarithmicmid-band or geometric mean frequency of the range considered, it being,of course, arranged that the cut-01f frequency is outside the band.Where the band is reasonably narrow the cut-off frequency can easily bearranged to be a long way outside the band.

An actual example of design will now be given; the values applying tothe two element section of Figure 8.

Frequency range :600-1400 k. c. Input impedance, Zoa=s2 Outputimpedance, Zob=6400n Impedance ratio=64/l Matching frequency, :=1000k.c.

No. of sections, 11:6 Taper ratio per section, B 7(64) =2 Z =Z0a /(1 B)=j.l00

1 B Y= g =yo005 The sections may then be built up into a chain withsuccessive values as shown in Figure 9. Realised in an actual structurethe line is as shown in Figure 10, the values of the inductances beingin microhenries and those of the capacities in micro-microfarads.

The "cut-off frequency for the low pass section having series inductanceL and shunt capacity C is derived from Z Y -1 i. e.

wfiLC' 1 where is the cut-ofi frequency.

If the matching frequency is m/271' Zm =jwmL Ym =jwmC where /(Zm Ym) Inthe above example i I 1000. 1414 k.c. a frequency lyingoutside therequired range.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, we declarethat what we claim is:

1. In an arrangement for connecting a dipole antenna of the center feedtype to remotely situated receiving apparatus, a concentric cable havingan inner conductor and an outer conductor said outer conductor beinggrounded, means including a plurality of inductances connected in seriesfor connecting one side of the dipole antenna to the inner conductor,the inductance values of said inductances being progressively greater inthe direction from the dipole toward the concentric cable, meansincluding a plurality of inductances in series for connecting the otherside of the dipole antenna to the outer conductor oi said cable, theinductance values of said last named inductances being progressivelysmaller in the direction from the dipole toward the concentric cable,and a plurality of condensers shunted between said two last named meansat various points along the lengths thereof, said condensers havingcapacity values which progressively decrease in the direction from thedipole toward the concentric cable.

2. In an arrangement for connecting a remotely situated receiver,capable of being tuned to receive both long waves and short waves, to along wave antenna and a short wave antenna through a common highfrequency cable, said cable having an inner conductor and an outerconductor, the outer conductor being grounded, a connection including aplurality of series inductances between said long wave antenna and theinner conductor of said concentric cable, the inductance values of saidseries inductances being progressively smaller in the direction from theantenna toward the cable, a plurality of shunt condensers of increasingmagnitude in the direction between the antenna and the cable connectedone thereof between ground and the aerial end of said connection and theremainder thereof between ground and the various junction points of saidseries inductances, a second connecting line connected between the shortwave antenna and said inner conductor and including a series ofcondensers of increasing magnitude in the direction between the shortwave antenna and the cable, a plurality of inductances of diminishingmagnitude in the direction between the antenna and the cable, onethereof being connected between ground and the short wave antenna andthe others being connected between ground and the junction points of thesaid series condensers.

3. In a signalling system the combination of a long wave antenna, ashort wave antenna and a receiver arranged so as to be tunable eitherover a band of long waves and over a band of short waves, a common highfrequency cable for feeding energy picked up from either of saidantennae to the common receiver, an artificial line for connecting oneend of said cable to the long wave antenna, said last named meanscomprising a plurality of filter sections of progressively difierentelectrical dimensions, said line matching the cable impedance at one endand the long wave antenna impedance at the other end, the impedancetransformation over the line taking place in a plurality of stepsdetermined by the number of filter sections in the line, a secondartificial line comprising a plurality of filter sections ofprogressively difierent electrical dimensions for cou pling the shortwave antenna to said end of the common cable, said second line matchingthe cable impedance at one end and the short wave antenna impedance atthe other end, the impedance transformation over the second line takingplace in a plurality of steps determined by the number of filtersections in the line.

4. Impedance transforming arrangement for matching the impedance of asource constituting a terminating impedance to the impedance of a loadconstituting another terminating impedance, one of said terminatingimpedances being balanced and the other thereof being unbalanced, a linecomprising a plurality of impedance transforming sections connected incascade between said terminating impedances, the section connected tothe balanced terminating impedance having substantially equal reactanceelements in both sides of the line, the section connected to theunbalanced terminating impedance having its series reactancesubstantially solely in one side of the line, the intermediate sectionshaving progressively different proportions of series reactance in thetwo sides of the line.

NoiiL MEYER RUST. JOHN FORREST RAMSAY.

